Marinate rainbow trout fillets in Italian salad dressing for 4 to 6 hours. Place fillets in a lightly greased microwave safe dish and discard salad dressing. Season with salt and pepper to taste. For added flavor add a light sprinkling of parsley or Italian herb seasoning. Place a small pat of butter on top if the fillets and cover lightly. Microwave on high for 3 to 4 minutes.